simple-scroll-hook

A simple, jQuery-less library for hooking HTML elements' CSS class changes to the window scroll. Multiple events can be bound to the scrollHook object, which creates only one scroll listener for everything hooked in. Additionally, after all its hooked events are fired, it cleans up after itself and removes its listeners to prevent memory leaks.

What are some use cases?

As of now, simple-scroll-hook works ideally as the user scrolls down a site.

Example use cases include fading in/sliding up text as the user scrolls into a new section, or making elements sticky as they scroll down.

Both can be seen in the example.html in this repo. To view, you can simply run the following in the command line:

open example.html

Installation

simple-scroll-hook can be used as:

1. A <script> tag, creating a window.scrollHook singleton object.
2. A Node.js / CommonJS module
3. An AMD module

Usage

Say we have these CSS classes:

.not-sticky {
  position: absolute;
  bottom: 0;
  right: 0;
}

.sticky {
  position: fixed;
  top: 0;
  right: 0;
}

And want to apply the .sticky class when the top of the viewport hits the top of <div class='not-sticky'>. We also want to remove .not-sticky.

Example as a module. var scrollHook = require('simple-scroll-hook'); can easily be replaced with var scrollHook = window.scrollHook; if used as a <script> global.

var scrollHook = require('simple-scroll-hook');


// Assume this exists in the DOM
var sticky = document.querySelector('.not-sticky');

// Transition states when top of viewport hits the top of this element
var transitionPos = sticky.getBoundingClientRect().top + window.innerHeight;

scrollHook.register(sticky, {
  initialStates: 'not-sticky',
  finalStates: 'sticky',
  position: transitionPos
});

// Register more transitions if desired

// Only call check for transitions every 250ms (Default 50ms).
scrollHook.setThrottleTime(250);

// This specific call will add the window listener
scrollHook.start();

That's it! Transitions should work as a user scrolls down, and simple-scroll-hook will automatically clean up after itself after all transitions are completed. This will also work if you have CSS3 animations in your classes.

Many of the functions are also chainable, so the above code could also be written as:

var scrollHook = require('simple-scroll-hook');


var sticky = document.querySelector('.not-sticky');
var transitionPos = sticky.getBoundingClientRect().top + window.innerHeight;

var exampleConfig = {
  initialStates: 'not-sticky',
  finalStates: 'sticky',
  position: transitionPos
};

scrollHook.register(sticky, exampleConfig)
          .register(anotherEl, anotherConfig);
          .register(yetAnotherEl, yetAnotherConfig)
          .setThrottleTime(250)
          .start();

API

scrollHook.register(element, config)

Register an element, and its configuration.

// Example configuration:
{
  initialStates: {string | array},  CSS class(es) of the element when out of view.
  finalStates: {string | array},    CSS class(es) of the element when scrolled into view.
  position: {int}                   (optional) Where to transition classes.
}

Note, simple-scroll-hook can act to just remove initialStates, making finalStates optional. It can also be done vice-versa.

position, if not specified, will default to the element.offsetTop.

scrollHook.setThrottleTime(ms)

Default 50ms. Set the interval between firing wheel event callbacks.

scrollHook.start()

Add the wheel event listener, and fire registered events.

Contributing

Issues and Pull requests are always welcome. Relevant test coverage is required, please.